One of the most important recent advances in spark ignition engines involves operation under lean conditions at low to moderate load to achieve fuel economy gains. Significant technological developments have been made in engine design and configuration to facilitate operation under lean conditions. Spark ignition engines are capable of operating with known fuels at a normalized fuel to air ratio (".PHI.") below 1.0. The normalized fuel to air ratio is the actual fuel to air ratio divided by the stoichiometric fuel to air ratio. The .PHI. at which an engine begins to exhibit unacceptable torque fluctuations is called the "lean limit". Still further fuel economy improvement in such engines may be achieved and NO.sub.x emissions reduced by operating the engine with a fuel capable of extending the engine's lean limit.
Fuel economy gains in these lean burn engines are typically realized during operation at low and moderate load; however at high load, these engines operate at a .PHI. of about 1, requiring that the fuel meet octane and other standard fuel specifications. Accordingly, to have practical application, the fuel of the present invention must meet octane and other standard fuel specifications.
Cold engine startup is a known source of problematic engine emissions. Spark injected ("SI") engines, lean burn or conventional, effectively operate under partially lean conditions during cold startup because of incomplete fuel vaporization. Lean limit improvements during cold engine start up would beneficially lower hydrocarbon emissions by reducing the fueling requirement for effective combustion.
There is therefore a need for a fuel that meets standard fuel specifications and is capable of extending the lean limit of engines. The fuel of this invention meets these needs.